This invention relates, in general, to sub-micron gate length semiconductor device fabrication, and more particularly to gallium arsenide MESFET fabrication with sub-micron gate lengths.
Optical photolithographic techniques used to manufacture semiconductor devices are limited in resolution due to aperture diffraction effects. For any given semiconductor photolithographic system, a balance must be achieved between the minimum feature size allowed on silicon and the ability to manufacture the minimum feature size in a production environment with an acceptable yield loss. Investment in new equipment having higher resolution can be prohibitive from a cost standprint.
Projection step and repeat technology is commonly used throughout the semiconductor industry. An I-line projection stepper which operates at a wavelength of 365 nanometer (nm) and has a 0.52 numerical aperture (NA) can form gallium arsenide (GaAs) MESFETs with 0.5 micron gate lengths under practical applications. Building smaller geometry MESFETs (less than 0.5 micron gate lengths) with this equipment may see a drastic increase in defects which translates into increased yield losses.
Other methods in research and development labs have been used to create small MESFET geometries. Examples of these techniques are E-beam lithography, DUV lithography, and process changes such as deliberate undercutting during gate metal etch. These methods have not been successfully implemented in a production environment at this time.
Gallium arsenide MESFET circuitry is commonly used in high performance systems. These high performance systems can only be built if MESFET speed is capable of meeting the performance criteria of the system. MESFET maximum operating frequency is a function of the gate length. For example, a MESFET having a 0.5 micron gate length will have an approximate maximum operating frequency of 18 Ghz. Typically, smaller gate lengths will increase MESFET maximum operating frequency. It would provide a great benefit if a MESFET which has a higher maximum operating frequency could be developed using existing I-line projection steppers and could be manufactured in a production environment.